I have been doing competitive programming for some months now. While attempting problems above 1800 in difficulty, I noticed that some of them required the use of ordered sets/multisets that support lookup, addition and deletion of elements in O(log(n)) time. Their solutions usually involve the use of std::set or std::multiset in C++. However, I code primarily in Python and there are no built-in packages available on Codeforces. Normally outside of Codeforces I just import sortedcontainers.sortedlist which I use like an Order Statistic tree. Here I discuss some strategies which I have used to overcome these issues. I hope that it helps others who are stuck so that they do not give up on their favourite languages. For C++ users, this may not be as useful, but you can still see how alternative data structures may be used to solve problems.↵
↵
The example I use here is CSES — Concert Tickets (for those with no CSES account, see [here](https://www.technicalkeeda.in/2020/10/concert-ticket-cses-problem-solution.html)). This question requires the deletion of entries, and searching for the next largest available entry in _sorted_ array **arr** smaller than a certain value **x**.↵
↵
**Method 1: Segment Trees**↵
---------------------------↵
↵
A segment tree is a very versatile data structure that work for cumulative searches for binary functions with associative properties. Go read up on it if you don't know what it is.↵
↵
For Concert Tickets, we need to find the largest non-deleted value closest to **x**. To do this, initialize a maximum segment tree with its values corresponding to its index value. i.e. the segmentTreeArray=[0,1,2,3,...n-1], and queries between [l,r] will give the largest value within the range [l,r]. Everytime we delete a value, we set the index corresponding to that value to -inf (or -1e9, if you prefer). This way, the index will not "interfere" with future query ranges that includes it. To find a value, first perform a binary search on the original array **arr** to find where the search should end such that arr[i]<x. To find the next largest available index, query for the largest from 0 to i.↵
↵
If you want to search for next smallest to the right, use a minimum segment tree, set value to inf (1e9) on delete, and search from i to n-1 instead.↵
↵
Complexity: O(log(n)) for search, insert, or delete. Adding elements is only possible if you know beforehand what are the elements you can add (and thereby allocate a "space" in your segment tree).↵
↵
What the code might look like:↵
↵
~~~~~↵
# class SegmentTree...↵
# def getMaxSegTree(arr) ...↵
def main():↵
↵
n,m=readIntArr()↵
↵
actual=readIntArr()↵
actual.sort()↵
custMax=readIntArr()↵
↵
availIndexes=getMaxSegTree(list(range(n)))↵
#if available, mark node as 1. Else, mark node as -inf↵
ans=[]↵
for cm in custMax:↵
b=n↵
i=-1↵
while b>0: # binary search↵
while i+b<n and actual[i+b]<=cm:↵
i+=b↵
b//=2↵
if i==-1: # no smaller value↵
ans.append(-1)↵
else:↵
idx=availIndexes.query(0,i)↵
if idx==-inf: # no available smaller value↵
ans.append(-1)↵
else:↵
ans.append(actual[idx])↵
availIndexes.updateTreeNode(idx,-inf) # delete the node↵
multiLineArrayPrint(ans)↵
↵
return↵
~~~~~↵
↵
Note: If you want to query element counts (i.e how many elements between 0 and i has not been deleted), use a sum Segment Tree and set index to 1 if element is present, and 0 if element is deleted. Then query(0,i) gives you the number of elements present between [0,i].↵
↵
↵
**Method 2: Linked List with Path Compression (like DSU)**↵
----------------------------------------------------------↵
↵
Note: From my (rather shallow) experience, this only works with item deletion.↵
Maintain an array **parent** in parallel to **arr** of same size. Originally **parent** looks like [0,1,2,...n-1]. Every time an item is deleted, replace parent[deleted_i] with deleted_i-1. When searching for a value, perform a binary search (like in segment tree example) for the index **i** such that arr[i]<x. Then, run a search for the parent of arr[i] with while(parent[i]!=i)i=parent[i]. Remember to perform a path compression after that, as this will make future searches faster.↵
↵
Note that if the search queries are for next larger item (item on right that isn't deleted), then replace parent[deleted_i] with deleted_i+1 on deletion.↵
↵
Complexity: O(1) for deletion. O(log(n)) for search, because of the binary search. I don't know the complexity of finding-of-parent-with-path-compression (see https://codeforces.com/blog/entry/2381). This linked-list with path compression runs quickly. For my submission for this question, it runs close to 2x faster than segment tree.↵
↵
What the code might look like:↵
↵
~~~~~↵
# no special classes needed :). only needs the parent array.↵
def main():↵
↵
n,m=readIntArr()↵
h=readIntArr() # n prices↵
parent=[i for i in range(n)] #price parent↵
t=readIntArr() # m customers↵
h.sort()↵
↵
ans=[]↵
for x in t:↵
b=n↵
i=-1↵
while b>0: # binary search↵
while i+b<n and h[i+b]<=x:↵
i+=b↵
b//=2↵
if i==-1:↵
ans.append(-1)↵
else:↵
eventualParent=i↵
while eventualParent!=-1 and parent[eventualParent]!=eventualParent:↵
eventualParent=parent[eventualParent]↵
if eventualParent==-1: # no smaller item found↵
ans.append(-1)↵
eventualParent+=1↵
else:↵
ans.append(h[eventualParent])↵
eventualParent-=1 # eventualParent is deleted. to update its parent to eventualParent-1 later↵
# path compression like DSU↵
while i!=eventualParent:↵
temp=parent[i]↵
parent[i]=eventualParent↵
i=temp↵
multiLineArrayPrint(ans)↵
↵
return↵
~~~~~↵
↵
↵
**Method 3: Actually using a self-implemented B-tree or Order Statistic Tree**↵
------------------------------------------------------------------------------↵
↵
This is straightforward. Just copy-paste already written code. For me, I copy-pasted the import library of sortedcontainers.sortedlist which is 800+ lines of code (eg. [submission:111203644]) after removing empty lines. I dislike this method because 1) it makes my code super long, 2) slows down my IDE which must parse all that code, 3) tends to have the largest overhead (usually slower than using segment tree). However, it doesn't require innovative thinking on how to manipulate a segment tree/linked list etc. to perform equivalent functions. Time complexity: O(log(n)) for search, insert and delete.↵
↵
What the code might look like: (looks elegant, but imagine I have 800+ lines of code for the OrderedList data structure. By the way, the **OrderedList** is my implementation of the Order Statistic Tree)↵
↵
EDIT: [user:rishabnahar2025,2021-03-31] has a good suggestion for an Order Statistic Tree template for Python users: [here](https://github.com/cheran-senthil/PyRival/blob/master/pyrival/data_structures/SortedList.py). This code is only 200+ lines long and runs faster than the OrderedList that I used previously. As pointed out by [user:xuanji,2022-03-30] in the comments, the documentation for the original sortedcontainers.SortedList implementation (for Python) mentioned that its operations run in n^(1/3) to n^(1/2) with small overheads. I should point out that I have used PyRival's SortedList to solve many problems requiring nlogn time complexity.↵
↵
~~~~~↵
# class OrderedList ... (800+ lines)↵
def main():↵
↵
n,m=readIntArr()↵
h=readIntArr() # n ticket prices↵
t=readIntArr() # m customers↵
↵
h.sort()↵
ol=OrderedList(h)↵
↵
ans=[]↵
for x in t:↵
idx=ol.bisect_left(x)↵
if idx==len(ol) or ol[idx]>x:↵
idx-=1↵
if idx==-1:↵
ans.append(-1) #cant take anything↵
else:↵
ans.append(ol.pop(idx))↵
multiLineArrayPrint(ans)↵
↵
return↵
↵
~~~~~↵
↵
↵
#### **Another Example**↵
↵
If you'd like to see these concepts used in another question, you may look at my solutions for [problem:1642E] where part of it requires marking the earliest visited times for items in indexes 1 to n, and each time a range is given (ranges may overlap).↵
↵
**Method 1: Direct update in using Lazy-Propagated Segment Trees** : [submission:151336538] (Note that I subsequently transferred the final result to a sparse table for faster range querying of the final result. This is out of scope for thisdiscussion so ignore that code)↵
↵
**Method 2: Linked list with path compression** : [submission:151410511]↵
↵
**Method 3: Using some sort of Order Statistic Tree (PyRival's SortedList)** : [submission:151340346]↵
↵
↵
Alright that's it. Thanks for reading this far, and I hope that this has been helpful or informative. If there're mistakes in my post please let me know.
↵
The example I use here is CSES — Concert Tickets (for those with no CSES account, see [here](https://www.technicalkeeda.in/2020/10/concert-ticket-cses-problem-solution.html)). This question requires the deletion of entries, and searching for the next largest available entry in _sorted_ array **arr** smaller than a certain value **x**.↵
↵
**Method 1: Segment Trees**↵
---------------------------↵
↵
A segment tree is a very versatile data structure that work for cumulative searches for binary functions with associative properties. Go read up on it if you don't know what it is.↵
↵
For Concert Tickets, we need to find the largest non-deleted value closest to **x**. To do this, initialize a maximum segment tree with its values corresponding to its index value. i.e. the segmentTreeArray=[0,1,2,3,...n-1], and queries between [l,r] will give the largest value within the range [l,r]. Everytime we delete a value, we set the index corresponding to that value to -inf (or -1e9, if you prefer). This way, the index will not "interfere" with future query ranges that includes it. To find a value, first perform a binary search on the original array **arr** to find where the search should end such that arr[i]<x. To find the next largest available index, query for the largest from 0 to i.↵
↵
If you want to search for next smallest to the right, use a minimum segment tree, set value to inf (1e9) on delete, and search from i to n-1 instead.↵
↵
Complexity: O(log(n)) for search, insert, or delete. Adding elements is only possible if you know beforehand what are the elements you can add (and thereby allocate a "space" in your segment tree).↵
↵
What the code might look like:↵
↵
~~~~~↵
# class SegmentTree...↵
# def getMaxSegTree(arr) ...↵
def main():↵
↵
n,m=readIntArr()↵
↵
actual=readIntArr()↵
actual.sort()↵
custMax=readIntArr()↵
↵
availIndexes=getMaxSegTree(list(range(n)))↵
#if available, mark node as 1. Else, mark node as -inf↵
ans=[]↵
for cm in custMax:↵
b=n↵
i=-1↵
while b>0: # binary search↵
while i+b<n and actual[i+b]<=cm:↵
i+=b↵
b//=2↵
if i==-1: # no smaller value↵
ans.append(-1)↵
else:↵
idx=availIndexes.query(0,i)↵
if idx==-inf: # no available smaller value↵
ans.append(-1)↵
else:↵
ans.append(actual[idx])↵
availIndexes.updateTreeNode(idx,-inf) # delete the node↵
multiLineArrayPrint(ans)↵
↵
return↵
~~~~~↵
↵
Note: If you want to query element counts (i.e how many elements between 0 and i has not been deleted), use a sum Segment Tree and set index to 1 if element is present, and 0 if element is deleted. Then query(0,i) gives you the number of elements present between [0,i].↵
↵
↵
**Method 2: Linked List with Path Compression (like DSU)**↵
----------------------------------------------------------↵
↵
Note: From my (rather shallow) experience, this only works with item deletion.↵
Maintain an array **parent** in parallel to **arr** of same size. Originally **parent** looks like [0,1,2,...n-1]. Every time an item is deleted, replace parent[deleted_i] with deleted_i-1. When searching for a value, perform a binary search (like in segment tree example) for the index **i** such that arr[i]<x. Then, run a search for the parent of arr[i] with while(parent[i]!=i)i=parent[i]. Remember to perform a path compression after that, as this will make future searches faster.↵
↵
Note that if the search queries are for next larger item (item on right that isn't deleted), then replace parent[deleted_i] with deleted_i+1 on deletion.↵
↵
Complexity: O(1) for deletion. O(log(n)) for search, because of the binary search. I don't know the complexity of finding-of-parent-with-path-compression (see https://codeforces.com/blog/entry/2381). This linked-list with path compression runs quickly. For my submission for this question, it runs close to 2x faster than segment tree.↵
↵
What the code might look like:↵
↵
~~~~~↵
# no special classes needed :). only needs the parent array.↵
def main():↵
↵
n,m=readIntArr()↵
h=readIntArr() # n prices↵
parent=[i for i in range(n)] #price parent↵
t=readIntArr() # m customers↵
h.sort()↵
↵
ans=[]↵
for x in t:↵
b=n↵
i=-1↵
while b>0: # binary search↵
while i+b<n and h[i+b]<=x:↵
i+=b↵
b//=2↵
if i==-1:↵
ans.append(-1)↵
else:↵
eventualParent=i↵
while eventualParent!=-1 and parent[eventualParent]!=eventualParent:↵
eventualParent=parent[eventualParent]↵
if eventualParent==-1: # no smaller item found↵
ans.append(-1)↵
eventualParent+=1↵
else:↵
ans.append(h[eventualParent])↵
eventualParent-=1 # eventualParent is deleted. to update its parent to eventualParent-1 later↵
# path compression like DSU↵
while i!=eventualParent:↵
temp=parent[i]↵
parent[i]=eventualParent↵
i=temp↵
multiLineArrayPrint(ans)↵
↵
return↵
~~~~~↵
↵
↵
**Method 3: Actually using a self-implemented B-tree or Order Statistic Tree**↵
------------------------------------------------------------------------------↵
↵
This is straightforward. Just copy-paste already written code. For me, I copy-pasted the import library of sortedcontainers.sortedlist which is 800+ lines of code (eg. [submission:111203644]) after removing empty lines. I dislike this method because 1) it makes my code super long, 2) slows down my IDE which must parse all that code, 3) tends to have the largest overhead (usually slower than using segment tree). However, it doesn't require innovative thinking on how to manipulate a segment tree/linked list etc. to perform equivalent functions. Time complexity: O(log(n)) for search, insert and delete.↵
↵
What the code might look like: (looks elegant, but imagine I have 800+ lines of code for the OrderedList data structure. By the way, the **OrderedList** is my implementation of the Order Statistic Tree)↵
↵
EDIT: [user:rishabnahar2025,2021-03-31] has a good suggestion for an Order Statistic Tree template for Python users: [here](https://github.com/cheran-senthil/PyRival/blob/master/pyrival/data_structures/SortedList.py). This code is only 200+ lines long and runs faster than the OrderedList that I used previously. As pointed out by [user:xuanji,2022-03-30] in the comments, the documentation for the original sortedcontainers.SortedList implementation (for Python) mentioned that its operations run in n^(1/3) to n^(1/2) with small overheads. I should point out that I have used PyRival's SortedList to solve many problems requiring nlogn time complexity.↵
↵
~~~~~↵
# class OrderedList ... (800+ lines)↵
def main():↵
↵
n,m=readIntArr()↵
h=readIntArr() # n ticket prices↵
t=readIntArr() # m customers↵
↵
h.sort()↵
ol=OrderedList(h)↵
↵
ans=[]↵
for x in t:↵
idx=ol.bisect_left(x)↵
if idx==len(ol) or ol[idx]>x:↵
idx-=1↵
if idx==-1:↵
ans.append(-1) #cant take anything↵
else:↵
ans.append(ol.pop(idx))↵
multiLineArrayPrint(ans)↵
↵
return↵
↵
~~~~~↵
↵
↵
#### **Another Example**↵
↵
If you'd like to see these concepts used in another question, you may look at my solutions for [problem:1642E] where part of it requires marking the earliest visited times for items in indexes 1 to n, and each time a range is given (ranges may overlap).↵
↵
**Method 1: Direct update in using Lazy-Propagated Segment Trees** : [submission:151336538] (Note that I subsequently transferred the final result to a sparse table for faster range querying of the final result. This is out of scope for thisdiscussion so ignore that code)↵
↵
**Method 2: Linked list with path compression** : [submission:151410511]↵
↵
**Method 3: Using some sort of Order Statistic Tree (PyRival's SortedList)** : [submission:151340346]↵
↵
↵
Alright that's it. Thanks for reading this far, and I hope that this has been helpful or informative. If there're mistakes in my post please let me know.
